There's a lot of talk about "alternative fuels" to gasoline and diesel these days, especially as national security concerns make reducing our dependence on foreign oil a priority. As the world wakes up to the climate crisis, it's become apparent that we need to do something drastic about our transporation -- which produces 28 percent of our emissions in the US -- to stem the greenhouse gas tide.

The truth is, not all of the "new" fuels are created equal. In fact, some are nearly as bad as gasoline in their environmental impacts, and others can't be scaled up in a meaningful way without creating other problems.

Here we show you the differences between the alternative fuels that have been generating headlines of late -- corn ethanol, hydrogen fuel cells, ultra-low-sulfur diesel, natural gas, cellulosic ethanol, biodiesel, and electric hybrid technologies.

Keep in mind that we don't recommend using even the best options available here forever. To truly avert a global climate crisis, we need to bring emission-free technologies to market by the end of the century. Replacing fossil fuels with the low-impact options available today is an important step forward as we work toward the day when every vehicle in the world produces zero emissions, and is fueled with renewable energy -- and our cities and towns are designed to encourage walking and biking.

Here's to driving less and pushing for ways to get the entire country to drive better.